Secondary batteries such as a lithium ion secondary battery are generally provided with an electrode plate group comprising a positive electrode, a negative electrode and a separator interposed therebetween. The separator serves to provide electrical insulation between the two electrodes and also to retain an electrolyte. Recently, in order to improve the product quality, it has been proposed to apply a slurry containing fine particles and a resin binder onto the surface of the electrode plate, and to dry the coated film with hot air to form a porous film, thereby integrating the electrode plate and the porous film (see the specification of Japanese Patent No. 3371301). A separator comprising a resin made of polyolefin and inorganic fine particles has also been proposed (see Japanese Unexamined Patent Publication No. Hei 10-50287).
In the case of forming a porous film, it is common to use a slurry obtained by dispersing the source material of the porous film in a liquid component using a disperser. The source material of the porous film contains fine particles that will serve as the filler in the porous film and a resin that will serve as the binder of the fine particles. The fine particles serving as the filler are supplied in the form of powder to the disperser.
Conventionally used fine particles comprise spherical or substantially spherical primary particles, and the primary particles are aggregated by weak van der Waals force (cohesive force), forming particle agglomerates. FIG. 3 shows a schematic diagram of a particle agglomerate 32 comprising conventionally used primary particles 31.
Therefore, when preparing the slurry, particle agglomerates are separated into primary particles as much as possible using a disperser such as a bead mill so as to stabilize the thickness and porosity of the porous film (see Japanese Unexamined Patent Publication No. Hei 10-106530 (FIG. 2)).